A previously known “neural dust” system includes small, implantable devices (referred to as “neural dust” or “motes”), an implantable ultrasound transceiver that communicates with each of the motes using ultrasound transmissions and backscatter transmissions reflected from the motes, and an external transceiver that communicates wirelessly with the implantable ultrasound transceiver. See Seo et al., Neural dust: an ultrasonic, low power solution for chronic brain-machine interfaces, arXiv: 1307.2196v1 (Jul. 8, 2013); Seo et al., Model validation of untethered, ultrasonic neural dust motes for cortical recording, Journal of Neuroscience Methods, vol. 224, pp. 114-122, available online Aug. 7, 2014; and Bertrand et al., Beamforming approaches for untethered, ultrasonic neural dust motes for cortical recording: a simulation study, IEEE EMBC (August 2014). The neural dust system described in these papers is used for cortical recording (i.e., the recording of brain electrical signals). In that application as shown in the papers, the motes are implanted in the brain tissue (cortex), the ultrasound transceiver (i.e., an “interrogator”) is implanted below the dura, on the cortex, and the external transceiver is placed against the head of the patient proximate to where the sub-dural ultrasound transceiver is implanted. This neural dust system is illustrated in FIG. 1.
Careful monitoring of certain physiological conditions in a subject can allow for a better understanding of health and disease prognosis. For example, blood sugar monitoring is used to monitor the health of a diabetic patient, and blood oxygenation levels are useful in monitoring compartment syndrome, cancer, or organ transplants. However, continuous deep tissue monitoring of certain physiological conditions is impractical using known technology. What is needed is an implantable device for sensing physiological conditions.